At standard conditions (1 atm, 25°C), volumetric and mass flow rates are nearly identical. However, rates differ dramatically with temperature and pressure changes.
For example, if volumetric flow was used to fill balloons with 250 mL of helium, but the incoming line ran near a cycling furnaceintermittently heating the incoming heliumthe balloons would initially be the same size, but would then be different sizes once they reached an equilibrium temperature. If a mass flow reading was used, the balloons would initially be different sizes, but would be the same size upon reaching an equilibrium temperature.
For applications at standard temperature and with no back pressure on the meter outlet, select a volumetric meter. For fluctuating pressures and/or temperatures, select a compensated mass meter.